Future high-power space missions will demand more solar array power with longer lifetimes. One method of increasing solar array power is the use of concentrating systems that increase the amount of incident sunlight for each solar cell. In the last three decades, several solar concentrator designs based on reflective, refractive, fiber-optic, and holographic systems have been developed. While some of these technologies have been developed mostly for terrestrial applications, with a promise of low-cost, high-levels of performance, and simplified manufacturing, few have been developed for space and on the move communication applications. Even for these systems, the solar conversion efficiency is low. In addition, for some solar power systems, the designs require solar tracking, specified power, lightweight packaging, and stiffness, all of which pose difficult engineering and design challenges.
Holographic optical elements or holographic gratings are holographically recorded holographic optical elements (HOE), by a wavefront reconstruction technique originally invented by Denis Gabor in 1948. The HOE functions as a lens and a mirror. Imaging properties have extensively been studied over the many years. The HOE manufacturing technique involves dividing coherent light from a laser into two equally polarized monochromatic waves of equal intensity where standing waves are created in the region where two waves overlap. A photosensitive plate in the overlap region records the intesity of the interference pattern for generating a holographic grating. The HOES can be volume or surface relief gratings. The volumetric HOE is where modulated light intensity creates a modulation of the index of refraction within the HOE volume. The surface HOE is where the interference pattern is projected on the surface. HOES can be transmissive or reflective types depending on how the interference patterns are recorded.
According to the Kogelnik theorem, transmission holographic diffraction efficiency is sine η=sin2 [v2+ξ2]1/2/[1+ξ2/v2] where ξ is a detuning parameter from the Bragg condition and v is a phase factor. For reflection holograms, the diffraction efficiency for polarization is given by η=[1+[1+ξ2/v2]/sin h2(v2−ξ2)]−1, where η is the efficiency. Current HOE materials are limited in wavelength sensitivity, thereby limiting applicable wavelengths and bandwidths. For most applications, λc and λr are different. This introduces aberrations where λc−λr≠0, and where λc is the HOE fabrication wavelength and λr is the application or playback wavelength. Inadequacies of current HOE systems limit their applications. The limitations include limited bandwidth, inoperability for 400>λ>700 nm, manufacturing problems, incident angular selectivity, and radiation safety. The operation and applicability of the solar HOE lies in extending the limited bandwidth of HOEs in comparison to the baseline solar spectrum and quantum efficiency of solar cells. In addition, a cost-effective technique is needed to extend the applicable λ to longer wavelengths.
In a paper by J. E. Ludman, J. R. Riccobono, H. J. Caulfield, T. D. Upton, “Solar Holography”, Holography: A tribute to Yuri Denisyuk and Emmett Leith, John Caulfield, Ed., Proc. SPIE, vol. 4737, p. 35, 2002, a solar holographic lens concentration and solar collector is taught having a single concentrator with a low bandwidth. The single separator and concentrator focus bands into serial photovoltaic (PV) cells. The concentrator does not separate the bands and then concentrates the band onto a single photovoltaic cell stack thereby having low conversion efficiency. The concentrator provides limited bandwidth and low power conversion efficiency. There are no means provided for constructing a complete commercial integrated device. The construction is not amenable to packaging a commercial device. The design is an inflexible HOE solar concentrator design. The device does not protect PV solar cells from harmful radiation including infrared, ultraviolet, gamma, and x-ray cosmic radiation, and does not enable solar radiation tracking from a wide range of incidence angles.
The HOE is inherently limited in its applicability. One problem is the aberration produced, because of wavelength shifts and dispersion. Another problem is low optical diffraction efficiency, and hence, low signal-noise-ratio, because of the inability to completely fulfill the Bragg condition. The shift in wavelength causes both longitudinal chromatic dispersion and geometrical aberrations. Therefore, when a HOE is used as a collector of an incident collimated multispectral or white light, the HOE disperses the beam in accordance to the Bragg condition, with limited bandwidths of the dispersed bands with undesirable optical aberrations.
Prism Solar Technologies produces a solar cell concentrator having a bandwidth for an unshielded PV cell and an HOE reflector displaced in a waveguide, so that sunlight is reflected and concentrated from the reflector to the PV cell with all components supported by a substrate. The waveguide is for receiving sunlight and routing the sunlight without band separation to the reflector with exiting light as transmitted light. The concentrator provides undesirable uncompensated aberrations such as dispersion and wavelength shifts produced by the reflector, so that the bandwidth of the reflected bands are not precisely matched to the band gaps of the PV solar cells, and therefore, the concentrator has limited bandwidths and low conversion efficiency.
U.S. Pat. No. 7,469,082, issued Dec. 23, 2008, entitled High Power Optical Fiber Laser Array Holographic Couplers, teaches a transmissive holographic optical element. The optical element includes a chirped grating that is used to inject light as side firing laser light from an array of laser diodes into an optical fiber with a reflective holographic optical element reflecting the injected laser light through and along the optical fiber with the transmissive holographic optical element HOE and reflective HOE sandwiching a portion of an optical fiber for increased laser side firing pumping efficiency in optical fibers.
Prior art solar holographic concentrators did not provide integral construction means for separation and concentration of optical light bands resulting in uncorrected and uncompensated optical aberrations producing low power conversion efficiencies. These and other disadvantages are solved or reduced by using the invention.